Question

State with equations what happens when borax is heated on a platinum wire Ioop and to the resulting transparent mass, a minute amount of `CuO` is added and the mixture is again heated first in the oxidising flame and then in the reducing flame of a Bunsen burner?

Answer 1

Correct Answer - When a borax is heated on a platinum wire loop, a transparent glassy beat called the borax bead is formed.
`Na_(2)B_(4)O_(7) . 10H_(2)Ounderset(-10H_(2)O)overset(Delta)toNa_(2)B_(4)O_(7)overset(740^(@)C)to ubrace(2NaBO_(2)+B_(2)O_(3))_("glassy mass");`
When this transparent glassy bead is heated with `CuO` in the oxidising flame, it imparts blue colour to the bead due to the formation of cupric metaborate.
`CuO + B_(2)O_(3) overset("oxidising flame")to Cu(BO_(2))_(2)`(blue bead)
However, when cupric metaborate is heated in the reducing flame of the Bunsen burner, the blue cupric metaborate is reduced by carbon present in the flame first to colourless coprous metaborate and then to metal which produces red scales.
`underset("Cupric metaborate")(2Cu(BO_(2))_(2))+C to underset("Cuprous metaborate")(2CuBO_(2)+B_(2)O_(3))+CO`
`overset(("blue"))(2CuBO_(2))+C to undersetunderset(("red"))("Copper metal")(2Cu overset(("Colourless"))(+)B_(2)O_(3))+CO`